Search

Premium Membership ♕

Limited Time Offer: Save 10% on Pro Plan with coupon: 110VDC. Study specialized LV/MV/HV technical articles & studies.

Home / Technical Articles / Three-phase basics and terms that students often mix

Three-Phase Basics & Hot Wires

We all know that an electric current needs a circuit (a complete circle) before it can flow. A single-phase wye circuit can be a phase and a neutral with the current flowing through the “hot” wire and returning to the source through the neutral. Most utility circuits consist of three phases.

Three-phase basics and terms that students often mix
Three-phase basics and terms that students often mix

It’s important to note that a three-phase circuit is not three different single-phase circuits, but one circuit with all three phases interconnected. Each phase helps to complete the circuit by acting as the return path for the other two phases.

A live conductor in a circuit tends to be called a phase because when three-phase voltages and currents are generated, each of the three conductors gets its voltage and current at a certain phase of a cycle.

An electrical phase is often represented in drawings and text by the Greek letter phi (Φ).

Ok, let’s get started with the characteristics of three-phase circuits, then continue with delta- and wye-connected electrical systems and conclude the article with three-phase power, calculations and examples.

Table of Contents:

  1. Characteristics of Three-Phase Circuits
    1. Why Three Phase?
    2. Generation of Three-Phase Power
    3. Phases 120 Degrees Apart
    4. Phase Designations
    5. Phase Rotation
    6. Three-Phase Connections
    7. Wye and Delta Systems
  2. Delta-Connected Systems
    1. Delta- or Series-Connected Three-Phase System
    2. Voltage in a Delta System
    3. Current in a Delta System
    4. Return Flow in a Delta Circuit
    5. Ground Fault Protection in a Delta Circuit
  3. Wye-Connected Systems
    1. Wye- or Parallel-Connected Three-Phase System
    2. Voltage and Current in a Wye System
    3. The Neutral in a Wye System
    4. Earth Return System Neutral
    5. Converting from Delta to Wye
  4. Three-Phase Power
    1. The Combined Power in Three Phases
    2. Field Calculations
    3. Calculations Involving Power
    4. Calculations with “Handy Numbers”
    5. Examples Using “Handy Numbers”

1. Characteristics of Three-Phase Circuits

1.1 Why Three Phase?

The effect of three-phase power as compared to single-phase power is similar to the effect of a six-cylinder engine as compared to a single-cylinder engine. A six-cylinder engine produces a smoother six small pulses per cycle while the single-cylinder engine produces one large pulse per cycle.

The values of the voltage and current in each of the three phases overlap with the other phases; therefore, three interconnected phases provide a smoother power than the relatively more pulsating power of a single phase. Three-phase current supplies a rotating magnetic field. Even though the power on each individual AC phase pulsates when it goes through the AC cycle, the sum of the power in all three phases at any point is constant.

Large generators and large motors are more efficient and are considerably smaller as three-phase units compared to equivalently powered single-phase units. In a three-phase motor, the magnetic field automatically rotates, bringing the rotor along with it.

With the same voltage and current per phase, a three-phase system needs only one additional wire (without a neutral, there is a 50% increase in conducting material) over a single-phase system but increases the circuit capacity by 73%.

A three-phase circuit can carry twice as much load as a single-phase circuit while maintaining the same voltage.

Suggested Course – AC Circuit Analysis: Fundamentals Course for Electrical Engineers

AC Circuit Analysis: Fundamentals Course for Electrical Engineers

Go back to the Contents Table ↑


1.2 Generation of Three-Phase Power

A simplified three-phase generator, as illustrated in Figure 1, shows three coils mounted on the armature at 120 degrees apart. Each coil generates an AC and voltage, but the power generated in each coil reaches its peak and direction at 120 degrees apart.

Commercial generators mount many coils on the stator and many magnets on the armature. The individual coils are wired so that they are connected together as three circuits 120 degrees apart. Each of the three circuits becomes a phase of a three-phase circuit.

Figure 1 – Simplified three-phase generator

Simplified three-phase generator
Figure 1 – Simplified three-phase generator

Go back to the Contents Table ↑


1.3 Phases 120 Degrees Apart

When three phases are 120 degrees apart, as shown in Figure 1, the values also can be shown in graph form as in Figure 2. The first vertical line in Figure 2 shows the values being generated in Figure 1.

Phase A, at 90 degrees, is generating at the maximum value, phase B, at 210 degrees is climbing towards the zero value, and phase C, at 330 degrees, is approaching the maximum return or negative value.

The second vertical line shows what is happening to the power generated in each phase when phase A is 120 degrees (one-third of the way) into the cycle, phase B would be 240 degrees (two-thirds of the way) into the cycle, and phase C would be at 360 degrees (at the end, which is also the beginning) of the cycle.

A three-phase circuit is like having three separate AC single-phase circuits with identical voltage that reach their peak values at a different time. At 60 hz, the second phase reaches its positive peak at 1/180 (0.00556) seconds after the time the first phase reaches a positive peak, and the third phase reaches its positive peak 1/180 (0.00556) second later. The first phase again reaches a positive peak 1/180 (0.00556) seconds after the third phase, starting the next cycle.

Even though each phase has the same voltage, they are out of phase with each other and there is a voltage difference between them.

Figure 2 – Relationship of three phases

Relationship of three phases
Figure 2 – Relationship of three phases

When the load on each phase is identical, the instantaneous power output of the three phases added together is constant. When one phase of a three-phase circuit reaches a peak voltage, another phase is close to zero volts and the third phase is on the return flow. As simple as it is!

So, each phase is 120 degrees out of phase with the other phase. The voltage and current in each phase are 1/180 second, or one-third distance in a cycle behind another phase.

Membership Upgrade Required

This content is not available in your premium membership plan. Please upgrade your plan in order to access this content. You can choose an annually based Basic, Pro, or Enterprise membership plan. Subscribe and enjoy studying specialized technical articles, online video courses, electrical engineering guides, and papers.

With EEP’s premium membership, you get additional essence that enhances your knowledge and experience in low- medium- and high-voltage engineering fields.

Limited Time Gift! – Save 10% on Pro Membership Plan with code 110VDC

Upgrade

Already a member? Log in here

Premium Membership

Get access to premium HV/MV/LV technical articles, electrical engineering guides, research studies and much more! It helps you to shape up your technical skills in your everyday life as an electrical engineer.
More Information
Edvard Csanyi - Author at EEP-Electrical Engineering Portal

Edvard Csanyi

Hi, I'm an electrical engineer, programmer and founder of EEP - Electrical Engineering Portal. I worked twelve years at Schneider Electric in the position of technical support for low- and medium-voltage projects and the design of busbar trunking systems.

I'm highly specialized in the design of LV/MV switchgear and low-voltage, high-power busbar trunking (<6300A) in substations, commercial buildings and industry facilities. I'm also a professional in AutoCAD programming.

Profile: Edvard Csanyi

Leave a Comment

Tell us what you're thinking. We care about your opinion! Please keep in mind that comments are moderated and rel="nofollow" is in use. So, please do not use a spammy keyword or a domain as your name, or it will be deleted. Let's have a professional and meaningful conversation instead. Thanks for dropping by!

  ×  3  =  12

Learn How to Design Power Systems

Learn to design LV/MV/HV power systems through professional video courses. Lifetime access. Enjoy learning!

EEP Hand-Crafted Video Courses

Check more than a hundred hand-crafted video courses and learn from experienced engineers. Lifetime access included.
Experience matters. Premium membership gives you an opportunity to study specialized technical articles, online video courses, electrical engineering guides, and papers written by experienced electrical engineers.